1. Field of the Invention.
The present invention relates to plotting mechanisms with the capability of selecting any of a plurality of pens mounted on a rotating carousel. More specifically, the invention relates to a computer controlled mechanism for indexing the pen carousel so that the desired pen is rotated to a reference position where the pen can be transferred to or from the drawing arm.
2. The Prior Art.
In the present era of automation, numerous devices are being developed that will accurately, efficiently, and quickly perform tasks that just a few years ago were performed manually. A large portion of the reason for automation is the rapid development of computers from the early room-sized devices to the present desk top personal computer. Microprocessor controlled devices such as the personal computer have greatly expanded capabilities for efficiently and accurately processing large amounts of data or other information. However, simply processing the data is not enough; the degree to which a computer is able to effectively convey data and other results of its processing to the user is a key measure of its utility to the user.
In recent years, graphical output has become an increasingly popular and commonly used form of output. For example, with presently available software, such as SPICE, an engineer can simulate complex electrical circuits without ever building them. Rather than presenting the results of such a simulation in a large table of numbers, it is much more desireable to present the output of the simulation in graphical form, such as a plot of output voltage vs. input voltage. This graphical output can be either sent to the screen of the CRT for viewing by the user, or to a plotting mechanism which makes a "hard-copy" of the graph on paper.
As another example of a plotting mechanism, Hewlett-Packard has developed a plotter than can be directly connected to a circuit in the same manner as an engineer would connect an oscilloscope. Thus a trained operator can easily obtain an accurate "hard-copy" of all the information viewable on a oscilloscope, including circuit characteristics such as voltage vs. time or output voltage vs. input voltage.
Plotter mechanisms can also be useful in the business world to provide graphical output such as stock trends against time, or to compare trends in various money markets against time. The output possibilities for plotter mechanisms are limited only by the size of the paper that can be inserted into the particular device. Many existing plotters allow the user to color various portions of his plot with a particular color of his choice. For example, an engineer may wish to compare several functional characteristics on same graph, and for that purpose he may select green for one characteristic, blue for another, and black for another. As another example a stock analyst may find it useful to have separate market trends appear in different colors. For example, the NYSE volume may appear in red, and the Tokyo Stock Exchange volume in blue. This capability for color selection is an important commercial element of virtually all plotters on the market today.
The mechanism for color selection typically includes a rotatable circular carousel having a plurality of pens affixed along its perimeter, each pen being a different color and also being removable from its position on the perimeter. Thus, when a certain pen is to be used for coloring a portion of the plot, a control device, often including a microprocessor, rotates the carousel until the chosen pen is at a transfer point where it can be grasped by a drawing arm. Then the chosen pen may be manipulated by the drawing arm under control of the plotter mechanism in order to draw the graph or other figure on paper or any other writing surface.
A key element of this color selection mechanism is the component for rotating the chosen pen until it is aligned with the transfer point. This component (or another) must also maintain the carousel in this aligned transfer position while the drawing arm latches onto the pen and removes it for drawing, and again it must maintain the carousel in the transfer position until the pen is deposited back into place on the carousel after the drawing arm has completed drawing with the chosen pen.
Components for rotating the carousel and positioning it at the aligned transfer position have taken various forms. Principally, precision devices such as stepper or servo motors have been used to both rotate the carousel and to align it with the transfer point.
A stepper motor has several windings at specific locations surrounding the rotor, each winding being switchable separately to independently connect each winding with a voltage source. A particular position of the rotor can be chosen by switching each winding either on or off. Thus, the number of switchable connections (drivers) must correspond to the number of separate windings. In order to move or rotate the stepper motor, a particular combination of switches are actuated, which applies a voltage to the associated combination of windings.
A servo motor most simply may be defined as an electric motor capable of providing feedback as to the position of the rotor, or one of its derivatives such as velocity. This feedback information is typically used to control the position of the rotor. In operation, a motor is rotated by application of a voltage while a control system monitors the feedback. When the control system determines that the rotor is in the desired position an appropriate signal is sent by the control device to stop motor rotation.
Either a servo motor and a stepper motor can be used to accurately position a carousel in a plotter mechanism. Using either type of motor, it is the position of the rotor that is used as feedback to control carousel rotation. The connection between the servo or stepper motor and the carousel maybe one of at least two types: (1) a direct connection, (2) a geared connection. In the direct connection, in each instance, it is a known position of the motor rotor that corresponds directly to the position of the carousel. In other words, the position of the motor rotor directly controls the position of the carousel. For example, if a carousel has six pens spaced equally around the perimeter of the carousel, each pen being numbered from one to six, and if pen 1 is that pen present at the transfer point, then to move pen 2 to the transfer point corresponds to a 1/6 rotation of the carousel and the stepper or servo motor.
Another type of connection between the servo or stepper motor and the carousel is the geared connection. In the geared arrangement, the output rotor of the motor turns a carousel through a series of gears of known ratio so that each increment of carousel rotation is associated with a known number of rotor rotations. If a servo motor is used in a geared arrangement, the feedback from the servo is monitored by the control device as the rotor turns. After a certain predetermined number of rotor rotations has occurred, the control device stops the motor.
Various factors influence reliability and cost of any electrical circuit or mechanical device. A simple circuit or device that utilizes few components and has fewer connections will in general be less expensive to produce and more reliable than a more complex circuit or device that performs the same function. Thus complexity of a circuit, including its connections and number of components, is a factor affecting cost and reliability. Furthermore, the extent of programming of a control device that is necessary to perform the desired objective influences the eventual cost and reliability of a product. Generally, a system with fewer types of inputs and outputs is easier to control than one with more numerous inputs and outputs. Thus, the variety of inputs and outputs affects the extent of programming necessary to accomplish a given objective.
As noted previously, to control a stepper motor, each winding must be switchable through an independent driver. If for example, a stepper motor has four windings, then the control circuit must be connected to four independent driver connections connected to the respective windings on the stepper motor, thereby affecting complexity of the circuit and the number of components therein. Furthermore, the control device may comprise substantial programming to, for example, generate the sequence to rotate a stepper motor three times.
Also, the servo motor may have a complicated connection between itself and the digital control circuit. If, for example, the servo motor provides analog feedback (i.e a variable voltage dependent upon position), then the analog feedback must be processed into a digital form before it can be used by the digital portion of the control device. This processing is performed by an A/D (analog/digital) converter which converts the voltage level to digital form (i.e. a series of ones and zeroes). Of course, the accuracy of the A/D converter is dependent upon the number of bits it can output. For example four bits of output can distinguish sixteen separate voltage values, each of which must have a separate connection with a digital portion of the control circuit. A quantization error is the amount by which the actual analog voltage may vary from digitally value. To reduce quantization error, an even larger number of bits must be provided as an output for the A/D converter, and there is a correspondingly greater cost and also a larger number of connections with the digital portion of the control device.
Some servo motors can directly provide a digital output indicative of position. For example, one commercially available servo motor provides a quadrature output of two bits indicative of the position of the rotor, the two bits providing a binary output of either 00, 01, 10, or 11, each of which indicates one of four positions of the rotor.
After a digital output has been provided to the control device, a digital state machine may be utilized to decode the output. This output may then be applied in a series of microprocessor-controlled operations to apply the position data to perform the desired objective. Furthermore a counter must be provided to count the number of rotor turns which must be compared with the predetermined number of turns in order to rotate the carousel to present the desired pen to the drawing arm. Thus, the processing of digital feedback from the servo motor includes decoding and counting, and therefore adds another layer of complexity to the programming of a digital portion of the control device. In addition, the servo motor requires use of a special component, a bidirectional driver in order to operate properly. A bidirectional driver is used because it is necessary to reverse polarity to the DC motor when the servo feedback is used to maintain the rotor in one position.
By nature, the stepper motor has certain disadvantages when compared with other DC motors. One disadvantage is that the stepper is less efficient than its DC counterpart. In other words, for a given amount of speed, a stepper motor dissipates more power and generates more heat. The hardware is also affected by this factor; each of the drivers must be of a high-power type in order to pass the larger amount of power necessary to operate the stepper motor.
A servo motor may incorporate a DC motor; however, the additional feedback circuitry adds another level of complexity to the motor. By adding a critical component to a DC motor, the servo motor becomes a precision device, thereby substantially increasing cost over that of a standard DC motor and also reducing reliability.
Finally, it must be noticed that both (stepper motors and servo motors) inherently are precision motor devices which must be manufactured under rigid standards in order to provide a reasonable degree of accuracy and reliability. When installed in a plotter mechanisms, both types of motors must be precisely positioned within the mechanism and precisely connected to the carousel.
Once physically installed, their numerous connections must be made with the respective components, such as drivers and the control device. Lastly, the digital portion of the control device must be programmed to accommodate the respective inputs and provide the outputs.
Thus, it would be an improvement in the art to provide a mechanism for rotating a carousel and for maintaining it in an aligned position which uses a low cost alternative to a stepper or a servo motor. It would be a further improvement in the art to provide such a mechanism that does not utilize feedback of rotor position to control the carousel rotation, and which has a simple input to the control device and is controlled by a simple output from the control device.